"His music is boring."

Translation: Jego muzyka jest nudna.

[JanLania]

Interesting that jego does not decline.

[Jellei]

Yup, it makes life a little bit easier. Same goes for "jej" (her) and "ich" (their).

[Schmoople]

Why is is this muzyka and not muzyki?

[lunaexoriens]

It is a normal, simple nominative sentence ([noun] is [adjective]) such as "Film jest nudny".

[manyclozel]

What about "jego muzyka to nudna"?

[Jellei]

Nope, doesn't work. You can only use the 'to' construction if there are noun phrases on both sides.